1. Technical Field
The present invention relates to a mold clamping apparatus for use in an electrically-operated injection molding machine, and more particularly, to a mold clamping apparatus having a toggle mechanism of a double toggle link type, which is capable of uniformly applying a pressing force onto a movable platen, and is low in cost and high in operational response.
2. Background Art
Typically, a toggle type mold clamping apparatus, mounted on an electrically-operated injection molding machine, includes a toggle mechanism having a pair of toggle sections which are disposed between rear and movable platens and are symmetric with each other in the vertical direction of the apparatus with respect to an axis of the same apparatus, the toggle sections being coupled to each other by means of a crosshead threadedly engaged with a ball screw which is disposed on the axis of the mold clamping apparatus. The mold clamping apparatus is so arranged that, as the crosshead is moved from its rear platen side toward the movable platen along the ball screw with rotation of the ball screws driven by a servomotor, two links of each of the toggle sections gradually expand to be switched from a state where there links are bent to another state where the links are stretched to establish a straight-line relationship therebetween, so as to apply a pressing force onto the movable platen. According to the mold clamping apparatus of this kind having the pair of toggle sections disposed symmetrically, it is possible to apply the pressing force generated by the toggle mechanism to the movable platen uniformly over the entire surface of the movable platen to a considerable extent. However, since primary parts of the mold clamping apparatus such as ball screw, crosshead, etc., are disposed and concentrated on the axis of the clamping apparatus, a flexure of the toggle mechanism is liable to occur, resulting in poor uniformity of the pressing force applied to the movable platen. Moreover, when a flexure of the ball screw occurs due to the weight of the ball screw and the weight of the crosshead and the like, this flexure produces forces which act on the upper and lower toggle sections in opposite directions (bending and expanding directions) from each other in respect of operational phases of these toggle sections, due to the vertically separated arrangement of the toggle sections. In this connection, the pressing force applied to the movable platen lacks uniformity. In this case, parallelism between the rear platen and the movable platen is deteriorated, and hence parallelism between the movable platen and the front platen is also deteriorated. Further, since a distance for which the pair of toggle sections are separated is large, the pressing force is applied to the movable platen at its peripheral portions separated from a central portion thereof, and hence the movable platen, in particular, a movable platen mounted with a small mold, is liable to be deformed, resulting in drawbacks such as the resultant molded products including burrs.